Persist and rebind feeds

- Adds Feed interface
- Adds abstractEntity.addFeed; if added like that then feed is persisted
- Adds EntityInternal.getFeedSupport(), which has getFeeds(), addFeed(),
  removeFeed() and getAllFeeds()
- AbstractFeed.poller: make private, with getter;
  and make transient
FunctionFeed: test persistence

- and make easier to build a FunctionPollConfig
Test rebinding SshFeed
Persist JmxFeed: testing and transient fields
Experimenting with Feed as EntityAdjunct
Tidy ChefAttributeFeed
Convert Feed to extend EntityAdjunct
Adds IllegalStateExceptionSupplier.EMPTY_EXCEPTION

- don’t create a new instance every time a Maybe.absent is constructed
Merge remote-tracking branch 'apache-gh/pr/108' into pr/108

Conflicts - to be resolved in subsequent commit
	api/src/main/java/brooklyn/entity/rebind/BrooklynObjectType.java
	api/src/main/java/brooklyn/entity/rebind/RebindContext.java
	api/src/main/java/brooklyn/mementos/BrooklynMemento.java
	api/src/main/java/brooklyn/mementos/BrooklynMementoManifest.java
	api/src/main/java/brooklyn/mementos/BrooklynMementoPersister.java
	core/src/main/java/brooklyn/entity/rebind/PeriodicDeltaChangeListener.java
	core/src/main/java/brooklyn/entity/rebind/PersisterDeltaImpl.java
	core/src/main/java/brooklyn/entity/rebind/RebindContextImpl.java
	core/src/main/java/brooklyn/entity/rebind/RebindManagerImpl.java
	core/src/main/java/brooklyn/entity/rebind/dto/BrooklynMementoImpl.java
	core/src/main/java/brooklyn/entity/rebind/dto/BrooklynMementoManifestImpl.java
	core/src/main/java/brooklyn/entity/rebind/dto/MementosGenerators.java
	core/src/main/java/brooklyn/entity/rebind/dto/MutableBrooklynMemento.java
	core/src/main/java/brooklyn/entity/rebind/persister/BrooklynMementoPersisterToObjectStore.java
	core/src/main/java/brooklyn/internal/BrooklynFeatureEnablement.java
This closes 108.

tidy up of 108, fixing merge conflicts and adding more places where Feed and CatalogItem need checks for persistence,
e.g. LookupContext and XmlMementoSerializer, and the (added in master branch) BrooklynMementoRawData